This invention relates to sealing devices and more particularly to a labyrinth seal for rotary heating equipment.
Rotary heating equipment such as kilns, furnaces, dryers, roasters, and ovens usually comprise a cylindrical structure that is slightly inclined to the horizontal and rotatable upon suitable bearings. The cylindrical structure generally includes a stationary or fixed end portion, and a rotatable portion communicating with the fixed portion. Solid feed introduced into the cylindrical structure normally progresses through it by virtue of rotation, pressure, and slope of the cylinder. The finished product generally discharges at the stationary end.
The necessary heat supplied to the feed or contents of rotary equipment can be provided by direct or indirect heat transfer methods. Direct heat transfer refers to the addition of heat by direct exchange between flowing gas and solid feed by means of, for example, a combustion chamber.
Indirect heat transfer is characterized by a separation of the heating medium from physical contact with the solids by means of, for example, metal walls or tubes.
A number of different methods have been employed to seal rotary equipment to prevent gas and/or dust from leaking between the rotatable and stationary portions of the rotary apparatus. One known type of seal employs a compression ring or bar mounted on a wearing pad to form a contact seal. See Perry & Chilton, Chemical Engineers' Handbook, 5th Edition, Section 20, page 38 (McGraw-Hill, 1973). Other known seals include complex interengaging metal members in the form of heavy castings or fabrications, usually spring biased or lever biased against each other and their respective mountings.
Contact type seals are subject to constant wear and must be periodically replaced. The maintenance and repair of such seals is a cumbersome operation due to the complex nature of the seal components, as well as the difficulty in aligning the interengaging members. Also, it is usually necessary to shut down the equipment in order to repair and replace the seals, leading to costly down-times.
Perhaps, the one common drawback to the multitude of known contact type seals approaches is that most of these sealing devices depend upon frictional contact to achieve their sealing effect. This approach has proven to be impractical in maintaining a proper seal in view of the poor rotation precision of rotary kilns, contributed to by thermal deformation and thermal elongation or contraction of the rotary kiln per se. Deflected loads can result, and are discontinuously imposed on the sliding surface of the seal mechanism. Furthermore, nonmetallic materials constituting the sliding face of the seal mechanism are subjected to high temperatures during the rotary kiln operation, resulting in the eventual breakdown and frequent replacement of the seal mechanism.
Efficient seals are essential for proper control and economical operation of rotary heating equipment. They reduce the entry of outside air. In some instances, complete containment of the enclosed system of rotating equipment is essential, as when toxic gases or solids are present. In other instances, entry of the external atmosphere into the apparatus could contaminate or react undesirably with the apparatus contents. It is also desirable for health reasons to prevent escape of dust and/or vapor from the equipment.
In contradistinction to contact type seals, labyrinth type seals perform a sealing function without frictional contact between the rotatable and the stationary portions of the heating equipment. One known type of labyrinth seal consists of an arrangement of axial rows of annular fins on the rotatable portion of the rotary kiln that mesh with alternately spaced axial rows of annular fins on the stationary portion of the kiln.
This type of labyrinth seal, although capable of preventing leakage, is not generally used on large diameter rotary equipment for a number of reasons. For example, due to the configuration of the fins, the outer cylinder must be of a split construction to enable its removal. In rotary equipment wherein cylinder diameters can exceed 10 feet, this type of construction makes it difficult to maintain proper alignment. Furthermore, labyrinth seals which have fins that are disposed axially, have little flexibility in terms of accommodating the angular and axial misalignment commonly encountered under operating conditions.